The present project investigates the structural aspect of interactions between myosin and actin (crossbridges) in muscle cells. (i) We characterized the binding of myosin to actin, i.e. formation of crossbridges, in the presence of ATPgS, a slowly hydrolyzable ATP analogue. The binding characteristics indicate that the ATPgS- myosin binds to actin strongly and the binding is cooperative in the absence of calcium. Thus the ATPgS-crossbridge state resembles those states accompanying or following the release of phosphate in the actomyosin ATPase cycle. This ATP analogue binds to myosin at nearly full saturation level, resulting in a pure ATPgS-crossbridge state, which could considerably facilitate studies of the strong binding states. (ii) Calculations showed that some of the features in the density maps reconstructed from five or seven equatorial reflections could be due to effects of insufficient terms used in the Fourier synthesis. However, through model calculations, it is possible to estimate, albeit approximately, the underlying density distributions derived from experimental data without the spurious features. (iii) Equatorial X-ray diffraction studies on intact and skinned cardiac muscle have been initiated. Preliminary results indicate that the interfilament spacing is inversely proportional to the amount of stretch beyond rest length, even in the skinned preparation.